Tyre pressure monitoring systems are well known. European Patent Application EP 0 793 579, which is hereby incorporated herein by way of reference, discloses an example of a known tyre pressure monitoring system. Typically, the system includes a pressure sensor which is incorporated into a vehicle's pneumatic tyre in order to monitor the air pressure therein. A control module, typically comprising a microprocessor, is provided for processing the output of the pressure sensor. The system also includes a transmitter for signalling the measured tyre pressure to a remote console usually provided in the vehicle's cabin. Normally, a battery is provided for supplying electrical power to the control module, the transmitter and any associated circuitry.
Conservation of battery power is an important consideration in a tyre pressure monitoring system. To this end, the system may adopt a normal operating mode when the vehicle is moving (or at least moving above a pre-determined threshold speed) and a conservation mode when the vehicle is stationary (or moving slower than a threshold speed). In the conservation mode, the system is less active than when in normal mode—for example, the pressure monitoring/transmitting cycle may occur one every few minutes rather then once every few seconds—and so battery usage is reduced.
Conventional tyre pressure monitoring systems usually include a mechanical roll switch (which typically includes a ball and spring mechanism), an accelerometer, or an equivalent mechanism, for detecting when the vehicle is in motion, or when the vehicle exceeds a threshold speed. Not only do such mechanisms add to the overall size of the system, but they are also considered to be relatively expensive and unreliable.
It would be desirable, therefore, to provide a relatively inexpensive, robust and space-saving alternative to conventional roll switches and equivalent mechanisms.
Several studies [1], [2] have indicated that a tyre when fitted to a wheel rim exhibits a natural resonance when excited. The presence of this resonance can be demonstrated when a fitted tyre is struck with a tyre iron or other object. The vibration or resonance of the tyre results in the generation of an audio signal or tone. The frequency of the tone is dependant on the structure and size of the tyre and also on the pressure (typically air pressure) inside the tyre. Research indicates that, for a normal passenger car, the general range of the resonant frequency is between 200 and 300 Hz.